This invention relates to the selective preparation of organic trisulfides. More particularly, it relates to a process for producing a substantially pure organic trisulfide from mercaptan and sulfur (or a high rank polysulfide as a sulfur source) wherein the sulfur is reacted with mercaptan in the presence of a heterogeneous catalyst which is an alumina-containing material, an alkali metal or alkaline earth metal modified silica, or a zinc oxide-containing material, as defined hereinafter. The process is operated either as a single step batch process, a continuous single-reactor process, or as a final step of a continuous multiple-step process, and may be characterized by the overall reaction illustrated by the following equation (1): ##STR1##
While the above equation demonstrates the stoichiometry of the reaction, surprisingly, in practice, an excess of mercaptan (RSH) is much preferred for efficient production of the pure trisulfide.
Organic polysulfides, for example, t-butyl polysulfides, t-nonyl polysulfides and the like, have excellent extreme-pressure (E.P.), anti-wear and anti-weld properties and are widely used as E.P. additives in lubricant compositions such as metal-working fluids and high pressure gear lubricants. However, polysulfides (RSS.sub.x SR, where x is greater than or equal to 2) exhibit high copper-strip corrosivity (ASTM Copper Strip Corrosion Test D-130-56), rendering them unsatisfactory for many commercial lubricating oil applications such as automotive and industrial gear oils. Organic trisulfides, on the other hand, have both optimum sulfur assay and low copper-strip corrosivity making them the ideal E.P. lubricant additives for applications where corrosivity to metals can not be tolerated.